1. Field of the Invention
The present invention relates to a method for cleaning an oil tank containing oil residues while simultaneously exploiting said oil residues, wherein the oil residues are fluidised to form a liquid mixture and wherein the liquid mixture thus formed is discharged from the tank and subjected to a treatment to separate the mixture into fractions.
2. The Prior Art
During the storage of oil products, such as crude oil, fuel oil and other refined oils, in oil tanks, a minor portion of the oil is precipitated. The precipitated material, the amount of which depends on the oil product type and origin, is deposited on the bottom and the sides of the oil tank and in time it will form a comparatively thick sludge layer in the tank. Especially in tanks used for the storage of crude oil, the layer thus formed will contain components other than hydrocarbons. Thus, there may be considerable amounts of water, sand, loose rust and sludge present in the layer.
When an oil tank containing such a sludge layer has been emptied, there is usually a water layer below the layer, and this water layer is discharged by pumping before the removal of the oil residue layer is initiated. The oil residue layer may be so solid that the removal thereof previously had to be carried out by manual or mechanical digging or by adding a major amount of another oil which was set in forceful motion by means of stirrers so as to suspend the oil portion of the sludge layer in the oil. This was done to facilitate the actual cleaning, which was performed manually, optionally with mechanical assistance, e.g., by use of sludge extractors or the like equipment.
U.S. Pat. No. 5,085,242 discloses a method of the type described above. In this known method a heating medium is introduced below the liquid surface through a manhole in an oil storage tank, which heating medium is preferably immiscible with oil and is preferably water in such an amount and at such a temperature that the oil residue in the tank melts and a liquid layer consisting of a lower water phase and an upper oil phase is formed at the bottom of the tank.
The removal of oil is preferably effected by positioning a floating pump in the tank, the inlet of the pump being located above the interface between the water phase and the oil phase. The oil is discharged by pumping through a second manhole in the oil tank and is introduced into a holding tank where heat is applied and it is stirred to prevent it from solidifying. While in the holding tank, water is separated off before the melted oil product, after renewed heating, is subjected to various after-treatments, such as filtration, decantation and centrifugation for separating off solid components, water residues and other undesired components. Following the discharge by pumping of the major part of the oil phase, the lower water phase and the solid particles precipitated at the bottom of the tank are discharged in a conventional manner, i.e., manually and/or mechanically.
According to a preferred embodiment of the known method, the heating medium used, as mentioned preferably water, is pumped out of the tank, and after filtration and renewed heating, it is recycled to the tank.